Replacing ceramic hob for induction - basic questions

[telectrix]

my apologies for previous post (since deleted my admin). full of cold, sneezing, eyes dripping like a tap. could hardly see keyboard. thought i had cut it.

 

[marconi]

Re#20 - fully magnetic pan or just magnetic base pan......

The majority of the magnetic interaction between the hob excitation coil and the pan is taking place in a thin surface layer at the bottom the pan's base. An alternating magnetic field does not penetrate far into a conductor and decreases in depth with increasing frequency of alternation.

The strength of the mutual coupling between the areas/regions on the pan and the hob coils reduces as they become farther apart so again the majority of coupling is to the base since it is flat and adjacent to the coils.

One can also visual that the base will 'shield' to some extent the upright sides of the pan thus reducing further mutual coupling.

Another consideration is that the regions in the sides into which are induced electromotive fields are shorter in dimensions or put crudely small circumference 'circles/bubbles of metal' ; the net electromotive force (emf) around such regions/paths is consequently of low value than in the longer paths/bigger regions of the flat base. Most of the magnetic/electric reaction then to the action of the exciting magnetic field is taking place in the base not the sides.

The large eddy current fluxes caused by the induced emf loops are in the base. And they flow in a thin skin region on the bottom surface. So because it is a thin conducting region the resistance to the electron flows/eddy currents is effectively higher than that of the bulk of the base; beyond the skin depth is in effect 'not used' for eddy current fluxes.

The base is then where the bulk of the interaction takes place between the alternating mag field and currents to flow in the skin layer. In the skin layer these current flows through the skin resistance produces Joule/Ohmic heating of the skin layer (IsquaredR). Thermal conduction of this heat through the pan and thence into its contents results in cooking.

I think the IH makers are working on even higher frequency excitation coils which would allow 'ordinary' copper, aluminium, non ferro stainless steel pans to be used.

The skin depth is approx given by :

What this tells us that the skin is thicker for materials with low permeability like copper and decreases in thickness with frequency. A thicker skin means lower R in IsquaredR so less Joule/Ohmic unless one can increase the size of the eddy currents by more powerful excitation fields.

Copper, Aluminium, non ferro stainless steel happen to be very good conductors and have very low permeability. To employ them usefully for IH cooking pans needs a higher frequency excitation field.


Skin depth in copper

Frequency	Skin depth (μm)
50 Hz	9220
60 Hz	8420
10 kHz	652
100 kHz	206
1 MHz	65.2
10 MHz	20.6
100 MHz	6.52
1 GHz	2.06

Producing high power excitation fields at higher frequencies requires more expensive power electronics than using a lower frequency (20-40kHz) and demanding 'special' pans to be employed.




Briefly, ferromagnetism adds to a pan's goodness for IH because of hysteresis losses manifest as heat.

Or something like this - it gives you the gist of what is going on.

 

[Zerax]

You beat me to it Marconi !

 

[rupertcacatua]

@marconi where is the emoticon for applause? Your post deserves it. Thank you for explaining it so elegantly sir. Hats off to you.

 

[SparkyHarry]

Great post @marconi